A number of networks have been developed and are being developed for delivery of interactive television, data and telephony services. A variety of delivery architectures, structures and mechanisms permit a wide range of functionality. For example, some networks, which offer only limited interactivity, employ existing coaxial cable infrastructure plus existing telephone lines for return path communications (upstream). Other networks employ Asymmetrical Digital Subscriber-Line ("ADSL") technology which forwards compressed digital video signals over existing twisted pair telephone lines at up to two Mb/s. A third broad category, the hybrid network, employs fiber optic links between the headend and a number of nodes which service neighborhoods of approximately 500 homes, and which connect to subscribers via coaxial cable links. In a fourth broad category, known as fiber-to-the-curb, fiber optic links pass the subscribers more closely and connect to subscribers via twisted pair loops. Fiber-to-the-curb networks are more expensive than the hybrid and ADSL networks, but are generally considered more robust and capable of accommodating greater interactivity because they are capable of accommodating greater bandwidth.
All of these fiber/coaxial/twisted pair networks and various versions of them involve physical cable plant to the home and thus considerable infrastructure investment, installation lead time, community disruption and customer annoyance. In addition, the cost of serving a customer is primarily a capital intensive sunk cost up front, since customers must be passed to be served. As existing cable designs demonstrate, systems without a substantial penetration and customer acceptance cannot be justified economically.
Such physical links also degrade over time due in part to moisture, temperature and flexure excursions, and the inherent environmental risks on the pole and in the ground. Such degradation can impose considerable efficiency losses over time, particularly as content suppliers require increasingly greater bandwidth capacity and efficiency.
Some see so-called "wireless cable" as a potential response to these issues presented by physical network infrastructures. Apart from the long lead time and expense required to pass hundreds of thousands of homes with wire line networks, wireless cable is impervious to physical degradation. In a conventional network topology, the "last mile" to the customer's home multiplied by the number of subscribers in the network comprises the vast proportion of the physical cable plant. Accordingly, replacement of that cable with wireless communications has received increasing attention in recent years.
One version of "wireless cable" networks is so called Multichannel Multipoint Distribution Service ("MMDS"). MMDS operates in the 2.1-2.7 Ghz microwave band, with a total of 33 analog 6-Mhz channels. Digital compression allows such MMDS networks with 33 analog channels to transport 100 to 180 digital programs. One entrant in the field has investigated combining MMDS with a telephone line return path for near video on demand capability. As a practical matter, it is apparent that with POTS return, full interactivity is probably not and never will be deployable due to the signaling logistics of TCP/IP networks.
Conventionally, however, MMDS has been viewed as providing insufficient bandwidth for full duplex interactive communications. Accordingly, certain entrants have investigated so called Local Multipoint Distribution Systems ("LMDS"). These operate at approximately 28 Ghz and follow the model of the cellular telephone. The high frequencies and concomitant greater bandwidth are tradeoffs for shorter transmission paths and thus smaller cell sizes. The higher frequencies also subject such networks to increased attenuation from foliage, weather, and other physical manifestations which obstruct the required line of sight path between the node and the subscriber station. In short, LMDS networks are viewed by some as lacking the robust RF link necessary to support emerging residential interactive video, data and telephony communications requirements which customers are demanding and which are becoming part of the required suite of services offered by entrants in this field. In order to obtain critical mass for coverage, LMDS cell sites would, at least in many parts of the country, be entirely too close to each other to be commercially and economically justified.